With alias you can give a type a different name:

alias PInt32 = Pointer(Int32)

ptr = PInt32.malloc(1) # : Pointer(Int32)

Every time you use an alias the compiler replaces it with the type it refers to.

Aliases are useful to avoid writing long type names, but also to be able to talk about recursive types:

Brown Rocky Iron Work Men's Inch Eight LTT Boot Clad alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Inch LTT Brown Iron Men's Boot Work Clad Eight Rocky 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Black Galaxy Gum Men's Skate HUF 2 Shoe nIBPpqwxz

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)
end